This invention relates to a printing press which includes multiple in-line central impression cylinders or drums and means for obtaining registration of the images which are printed on the cylinders.
Currently, high quality flexographic printing using extensible web materials is done on central impression (CI) presses. The flexographic process offers distinct cost advantages and flexibility compared to other types of printing such as gravure. The advantage of using a CI drum over an in-line flexopress is that the web is stable during the printing process. The stability of the web allows for higher quality printing on a variety of substances including extensible films. Registration error can be maintained to within 0.003 inch between any two colors being printed.
Unfortunately, the number of colors that can be printed on a single CI drum is limited by a number of factors, including:
a) size of the drum (manufacturability of the drum decreases as size increases); PA1 b) the maximum repeat of the plate roll; PA1 c) size of the dryer between the color decks; PA1 d) space required for operator to view the web; PA1 e) access to the decks (i.e. safety hazards exist as operator climbs on the machine); PA1 f) overall height of machine (exceeds ceiling height in some plants). With these considerations, a maximum of 10 colors is realistic, but still troublesome. PA1 a) Deformation of the web caused by heating the web as it travels through a dryer section located between the press sections. PA1 b) Deformation of the web caused by the tension required to transport the web from the first press section to the downstream sections. The web path between press sections is long since the web travels away from the press and into a tunnel dryer before turning back toward the next press section. The longer the web path, the greater the amount of idler roll friction the web needs to overcome or drive, which in turn is proportional to the amount of tension imparted on the web. PA1 c) Transmission errors and backlash in the drive train caused by the pitch variation of the associated gearing. PA1 a) Limited compensation. The unit can run out of adjustment stroke when the register error is continually in one direction. PA1 b) Compensation unit takes up space in the machine. The unit must be mounted between the press sections where there is limited physical space. PA1 c) Excessive web wrap around compensator idlers, which requires web to be completely dried or picking of the ink will occur. PA1 d) Operator needs to adjust system to maintain correct responsiveness to the system, especially when material is changed. PA1 e) Operator needs to manually obtain correct registration before registration controller is enabled. Again. this requires highly qualified personnel and takes time. PA1 a) If registration error is continuous in one direction and if registration is adjusted with tension, extreme tensions (high or low) can be built which can cause web breaks. PA1 b) Operator needs to manually obtain correct registration before registration controller is enabled. PA1 c) Limited accuracy of the registration correction command. The system adjusts the registration by adding a trimming signal to the velocity command of the drives. Errors can occur in the conversion from a position-based signal to a velocity signal.
Tandem in-line presses have been built before. For example, applicants' assignee Paper Converting Machine Company built a tandem press in 1989 that used two 4 color stations in-line. One of the purposes of the press was to decrease changeover time by running a job on one of the press sections while the other section was changed over to run the next job. once the first job was done, the press that contained the next job would run. The changeover time was decreased, but added cost was incurred by having two press sections when only one press section was required.
Another purpose of the tandem design was to improve operator access by using smaller press sections. However, the resulting compromise, when multiple press sections were required to print, was increased registration error between any two printed colors. The color-to-color registration is normally between 0.015 inch and 0.030 inch, depending upon variables such as machine speed, web material, and dryer temperature. These variables create a number of conditions, including:
Further problems with this design included the cost associated with having and supporting two overhead dryer sections (one between press sections and one after the last press section). Furthermore, the machine is quite sensitive to operator adjustments. To obtain the desired print, the operator had to adjust tensions, dryer temperatures, drum temperatures, and registration control parameters. This caused longer make-ready times compared to single CI presses and required highly qualified and trained personnel. Consequently, the limitations in this design resulted in high operating costs.
In virtually all in-line designs, including flexo or gravure, a registration system is used to maintain print registration from one press section to the next. The common method for registration control is to use a web compensation system. A web compensator adjusts the length of the web path and ultimately the tension within the zone between the two press sections. An isolated system furnished by a number of vendors such as Bobst and Eltromat measure the registration error and control the web path length. Problems with this type of system include:
Another method to control print register is to modify the command to the sectionalized drives with an adjustment based upon the registration error. Depending upon the method used, the tension of the material between the press sections may be affected. Otherwise, the printing cylinders are adjusted by increasing or decreasing their velocity based upon the amount of registration error. This method eliminates some of the challenges associated with a web compensator, but it still has limitations: